1. Field of the Invention
This invention relates to magnetic disc devices, and more particularly to magnetic disc devices for recording and/or reproducing information on magnetic discs having no index hole for use in detecting the absolute address. Still more particularly it relates to improvements in the means for indicating and detecting the absolute address of the magnetic disc.
2. Description of the Prior Art
Floppy discs or like magnetic discs have recently found wide use as the external memory in computers, word processors or like information processing apparatuses, and are available in many sizes of 8 inch, 5 inch, 3.5 inch and so on. Of these, the 8 inch and 5 inch magnetic discs have an index hole bored therein. The position of this hole is detected by a light sensor of the magnetic disc device to obtain an index signal. Thus the absolute address of the magnetic disc can be indicated and detected.
For 3.5 inch magnetic discs, on the other hand, because of their generally having no such index hole, the above-identified method is no longer useful for getting the index signal. On this account, another method has been employed that the motor and the disc are made chuck at a certain angle, so that the detection of the angular position of the rotor of a spindle motor for driving rotation of the magnetic disc leads to indicate and detect the absolute address of the magnetic disc.
To detect the particular angular position of the rotor, use may be made of a Hall element, a Hall IC, a photo-sensor, or like sensing device. Of these, the Hall element has been widely accepted in the magnetic disc devices because it admits of relatively low in cost and simple in the structure of construction.
FIG. 1 shows an example of the prior known magnetic disc device of this kind, wherein a spindle motor 1 has a small magnet piece adhered to the rotor 9 thereof. A Hall element 3 for detecting the small magnet piece 2 is connected between current limiting resistors 11 and 12. An IC 6 for driving the motor 1 is connected to a pair of stator coils 7 and 8 of the motor 1. Another IC 4 controls the operation of the motor 1 in accordance with information representing the angular position of the rotor 9 sensed by the Hall element 3.
It should also be recognized that because the output voltage of the Hall element 3 is as weak as 5 to several hundreds of millivolts, the ordinary control IC 4 is difficult to treat it as it is. So, the magnetic disc device has conventionally been provided with an amplifier circuit 5 in the form of a separate IC unit from the above two ICs 4 and 6 as arranged between the Hall element 3 and the control IC 4. This implies that the number of parts increases, the consumption of electrical power is intensified and the price of the device rises. A large obstacle has been thrown in the way of advancing minimization of the size of the magnetic disc to conform with the reduced size of the magnetic disc.
Another kinds of devices using the above-mentioned Hall IC or photo-sensor, though their not requiring the addition of a particular amplifier circuit, have an alternative problem that because these detecting members are more expensive than the Hall element, it becomes impossible to avoid an objectional increase in the price of the magnetic disc device.